The Multiverse is the collection of all three (and lower) dimensional universes in existence. This includes all universes that have ceased to exist, as well all those that will exist. Some of these universes differ only slightly to our own, and others are entirely incomprehensible. Life is known to exist in at least a few of these worlds, notably the Antiverse.
Little is known in regards to what exists beyond the third dimension. Although challenges like time and distance are relatively trivial in the third dimension, higher dimensions pose tremendously difficult obstacles in research.
At the time being, LOGO is yet to agree to disclosing more information to me, but he doesn't deny the existence of virtually infinite worlds even more baffling than the ones in the Grand Archives.